The present invention relates to nonwoven, aromatic polyamide sheet structures and methods for making such sheet structures and more particularly relates to thin, aromatic polyamide paper with a thickened area along its edges to facilitate calendering and a method for its manufacture
Nonwoven, aromatic polyamide sheet structures such as the poly(meta-phenylene isophthalamide) papers sold under the trademark NOMEX by E. I du Pont de Nemours and Company are useful in a variety of applications requiring resistance to thermal degradation and/or electrical insulative properties. For many such applications, papers which have been calendered at high temperatures and pressures are advantageously employed due to their enhanced mechanical properties. However, thin aromatic polyamide papers, for example, uncalendered thickness less than about 150 microns, are difficult to calendar. Frequently, "work roll wraps" occur, that is, papers adhere to the hot calendar rolls and wrap around the entire work roll causing a sheet break. Work roll wraps can thereby cause loss of a product and significant downtime in the calendering process.
For thin aromatic polyamide papers, one of the primary causes of calendar work roll wraps is excessive fibrid which clings loosely to short fibers (floc) that protrude from the edge of the formed sheet. As the sheet is passed through the high temperature/high pressure calendar work roll nip, the "loose" floc and fibrid at the sheet edges adheres to the work roll and build up a significant deposit. Once the deposit builds up enough and the work roll oscillates the deposit back into the sheet edge, the thin papers can "stick" to and wrap around the roll. Since thin papers are not as stiff and strong as thicker papers, sheet breaks result frequently. For example, in the calendering of 130 micron (5 mil) papers with 250 fibers per centimeter protruding from the edges, calendar roll wraps can occur as frequently as 1 every 12,000 meters.
Accordingly, thin aromatic polyamide sheet structures which are readily calendered and which do not have a propensity for work roll wraps would be highly desirable.